The present invention relates to the manufacture of integrated circuits. More specifically, the invention relates to an improved method of depositing silicon oxide layers for use as a dielectric material in such integrated circuits.
One of the primary steps in the fabrication of modern semiconductor devices is the formation of a film, such as a silicon oxide, on a semiconductor substrate. Silicon oxide is widely used as an insulating layer in the manufacture of semiconductor devices. As is well known, a silicon oxide film can be deposited by thermal chemical vapor deposition (CVD) or a plasma-enhanced chemical vapor deposition (PECVD) processes. In a conventional thermal CVD process, reactive gases are supplied to the substrate surface where heat-induced chemical reactions (homogeneous or heterogeneous) take place to produce a desired film. In a conventional plasma process, a controlled plasma is formed to decompose and/or energize reactive species to produce the desired film.
Semiconductor device geometries have dramatically decreased in size since such devices were first introduced several decades ago. The dramatic reduction in device geometry has, in turn, dramatically increased the density of circuit elements and interconnections formed in integrated circuits fabricated on semiconductor substrates. One persistent challenge faced by semiconductor manufacturers in the design and fabrication of such densely packed integrated circuits is the desire to prevent spurious interactions between circuit elements.
Unwanted interactions are typically prevented by providing spaces (sometimes referred to herein as a “gap” or “trench”) between adjacent elements that are filled with an electrically insulative material to isolate the elements both physically and electrically. As circuit densities increase with smaller feature sizes, the widths of these spaces decrease, increasing their aspect ratio. The aspect ratio of a gap is defined by the ratio of the gap's height or depth to its width. High aspect ratio gaps are difficult to fill using conventional CVD methods. A film's ability to completely fill such gaps is referred to as the film's “gap-filling” ability. Silicon oxide based films (also referred to as “silica glass” based films) are one family of insulation films that is commonly used to fill the gaps in intermetal dielectric (IMD) applications, premetal dielectric (PMD) applications and shallow trench isolation (STI) applications among others. A silicon oxide film used in such an application is often referred to as a gap-fill film or a gap-fill layer.
Some integrated circuit manufacturers have turned to the use of high density plasma CVD (HDP-CVD) systems to deposit silicon oxide gap-fill layers. HDP-CVD systems form a plasma that is approximately two orders of magnitude or greater than the density of a standard, capacitively-coupled plasma CVD system. Examples of HDP-CVD systems include inductively-coupled plasma systems and electron cyclotron resonance (ECR) plasma systems among others. HDP-CVD systems generally operate at lower pressure ranges than low density plasma systems. The low chamber pressure employed in HDP-CVD systems provides active species having a long mean-free-path and reduced angular distribution. These factors, in combination with the plasma's density, contribute to a significant number of constituents from the plasma reaching even the deepest portions of closely spaced gaps, providing a film with improved gap-fill capabilities as compared to films deposited in a low density plasma CVD system.
Another factor that allows films deposited by HDP-CVD techniques to have improved gap-fill characteristics as compared to films deposited by other CVD techniques is the occurrence of sputtering, promoted by the plasma's high density, simultaneous with film deposition. The sputtering element of HDP deposition slows deposition on certain features, such as the corners of raised surfaces, thereby contributing to the increased gap-fill ability of HDP deposited films. Some HDP-CVD systems introduce argon or a similar heavy inert gas to further promote the sputtering effect. These HDP-CVD systems typically employ an electrode within the substrate support pedestal that enables the creation of an electric field to bias the plasma toward the substrate. The electric field can be applied throughout the HDP deposition process to further promote sputtering and provide better gap-fill characteristics for a given film.
It was initially thought that because of their simultaneous dep/etch nature, HDP-CVD processes could fill the gaps or trenches that were created in almost any application. Semiconductor manufacturers have found out, however, that there is a practical limit to the aspect ratio of gaps that HDP-CVD films are able to fill. For example, one HDP-CVD process commonly used to deposit a silicon oxide gap-fill film forms a plasma from a process gas that includes silane (SiH4), molecular oxygen (O2) and argon (Ar). It has been reported that when such a process is used to fill certain narrow-width, high aspect ratio gaps, the sputtering caused by argon in the process gas may hamper the gap-fill efforts. Specifically, it has been reported that material sputtered by argon in the process redeposits on the upper portions of the sidewalls of the gaps being filled at a rate faster than at the lower portions. This in turn, may result in the formation of a void in the gap if the upper areas of regrowth join before the gap is completely filled.
FIGS. 1A-1C, which are simplified cross-sectional views of a silicon oxide film at different stages of deposition, illustrate the potential gap-fill limitation that is associated with some HDP-CVD processes that are required to fill certain small width gaps having relatively high aspect ratios. The gap-fill problem illustrated in FIGS. 1A-1C is somewhat exaggerated in order to better illustrate the problem.
FIG. 1A shows the initial stages of film deposition over a substrate (not shown) having a gap 10 defined by two adjacent features 12, 14 formed over the substrate. As shown in FIG. 1A, the conventional HDP-CVD silicon oxide deposition process results in direct silicon oxide deposition on horizontal surface 16 within gap 10 and horizontal surfaces 18 above features 12, 14. The process also results in indirect deposition (referred to as re-deposition) of silicon oxide on sidewalls 20 due to the recombination of material sputtered from the silicon oxide film as it grows. In certain small-width, high-aspect-ratio applications, the continued growth of the silicon oxide film results in formations 22 on the upper section gap sidewall that grow toward each other at a rate of growth exceeding the rate at which the film grows laterally on lower portions 24 of the sidewall (see FIG. 1B also). The final result of this process is that a void 36 forms as shown in FIG. 1C
A variety of techniques have been developed to extend the gap-fill capabilities of silica glass HDP-CVD processes. Two specific examples include, U.S. Pat. Nos. 5,872,058 and 6,395,150, each of which is assigned to Novellus. The '058 patent discloses that the gap-fill capabilities of a silica glass film can be extended by reducing the amount of argon or other inert components in the HDP oxide process. Specifically, the '058 patent teaches that in order to fill higher aspect ratio gaps, it is important to limit the amount of inert components in the process gas to no more than 13 percent of the total gas mixture.
The '150 patent discloses that if argon, which is a diluent gas in addition to a sputtering agent, is eliminated from the process gas as suggested in the '058 patent, deposition rate uniformity may suffer. The '150 patent teaches this problem may be addressed by adding a flow of helium to the process gas. The '150 patent teaches that the amount of helium added should be between 15-90 percent of the total gas mixture and lists between 10-2000 sccm as an exemplary flow rate range. The '150 patent also lists hydrogen as an example of a diluent that can be used instead of helium. The '150 patent does not disclose specific details about what gaseous sources may be used to supply the hydrogen or how much hydrogen should be added.